Euler was a leading mathematician in the 18th century, and is considered to be one of the greatest of all time. He wrote prolifically on topics as diverse as number theory, analysis, applied maths and physics. He was also a formidable mental calculator. He was able to disprove one of Fermat’s conjectures that all numbers of the form 22n + 1 are prime by finding that 4,294,967,297 is divisible by 641. After 1766 he became blind, but his last years were the most productive of all.